1. Field Of The Invention
The present invention relates generally to AC drive power conversion systems and, more particularly, to an apparatus and method for high slip operation of an AC electric motor at substantially zero rotation and substantially zero torque, which electric motor being supplied an outgoing signal, such as a drive current, of variable magnitude and frequency by an AC electric motor drive system.
2. Prior Art
Direct current (DC) motors have traditionally been used in electric drive systems to produce a mechanical rotation over a variable rotation range at substantial torque levels. However, DC motors exhibit several major deficiencies, including high maintenance costs and radio frequency interference problems caused by arcing and concomitant mechanical deterioration of the brushes used in such motors.
The trend in recent years has been to use AC motors in electric drive systems which produce variable mechanical rotation of substantial torque. AC motors are attractive technically and commercially because of their lack of brushes and inherent ruggedness of design.
An excellent analysis of the theory and operation as well as the attributes and deficiencies of DC and AC motor types is found in Electrical Mechinery, the Processes, Devices and Systems of Electromechanical Energy Conversion, 3d Ed., by A. E. Fitzgerald et al, McGraw-Hill Book Company, New York, 1971.
One type of AC motor is the AC induction motor. The AC induction motor has been used in AC drive power systems for producing a variable mechanical rotation of substantial torque.
In such drive systems, the AC induction motor produces a variable mechanical rotation of variable torque in response to an outgoing signal, such as a drive current, of variable magnitude and frequency. This drive current typically is supplied from a variable frequency inverter. The inverter converts a DC current of controllable magnitude into the drive current of variable magnitude and frequency; in the case of the thyristor inverter, the drive current is generated as a result of controlled gating of the thyristors. The inverter typically has commutating capacitors used to commutate automatically the thyristors. This automatic commutation produced by the commutating capacitors requires, however, a charge of appropriate magnitude and polarity on each commutating capacitor.
The DC current of controllable magnitude provided to the inverter can be supplied from any DC current source, but typically is provided by a DC converter via a DC link having an inductor.
A conventional drive system utilizing an AC induction motor typically can provide substantially zero rotation at substantial torque. One way this can be accomplished is by operating the induction motor in a "constant slip" mode. The concept of slip is explained in detail below, but it is sufficient for present purposes to state that per-unit slip s is expressed as s=(n.sub.l -n)/n.sub.b, where n is the rotation produced by the rotor of the motor in revolutions per minutes (rpm), n.sub.l is the synchronous rotation of the stator field of the motor in rpm, and n.sub.b is the synchronous speed of the stator field at motor rated rpm. In this regard, reference is made to pages 188-89 of the Fitzgerald, et al reference presented above.
When the drive system produces zero rotation at substantial torque by operating the induction motor in a constant slip mode, the amount of generated torque is controlled by varying the magnitude of the drive current. To produce the substantial torque, the per-unit slip must have a very low value, for example, 0.02, in order to operate the induction motor in the required region of its torque-slip curve.
Rapid acceleration of the motor rotor out of the zero rotation, substantial torque mode is possible for three reasons. First, a sufficient charge of proper polarity is maintained on each commutating capacitor because the frequency of the drive current is very low, for example, 1 to 2 Hertz (Hz), but is not 0 Hz, and the magnitude of the drive current is high. Secondly, the flux level in the motor needed to generate torque is high due to the high magnitude of the drive current. Lastly, the magnitude of the current flowing through inductor of the DC link is already high, thus not requiring a substantial rate of change of current.
Conventionally, when it is desired to operate the AC induction motor in the substantially zero rotation and substantially zero torque mode, the magnitude and frequency of the drive current are reduced substantially to zero values. This results in several problems in system performance. In high performance drive systems, it is essential that the AC induction motor be able to accelerate rapidly on command from the substantially zero rotation and substantially zero torque condition. However, this rapid acceleration is not presently possible in existing AC electric motor drive systems because of two inherent problems.
First, in order to produce substantially zero rotation and substantially zero torque, the frequency of the drive current must be at a very low value, typically zero Hz. This is shown by FIG. 4, which plots slip on the vertical axis with respect to torque on the horizontal axis. This low frequency value, however, causes the requisite charge on the commutating capacitors to bleed off because the inverter is not being commutated. The insufficient charge on the commutating capacitors results in unsatisfactory commutation when the drive system is rapidly taken out of the substantially zero rotation and substantially zero torque condition.
The second problem is that the required reduction in the magnitude of the drive current causes the level of the DC current in the DC link connecting the inverter with the DC current source to be at a low level. As stated above, the DC link typically includes an inductor connected in series between the DC current source and the inverter. As is well known, the current through an inductor cannot be changed instantaneously; instead, a finite amount of time is required to raise substantially the level of the current flowing through the inductor. Thus, a time delay is also introduced in a conventional system when the system is taken out of the substantially zero rotation and substantially zero torque condition due to the inductor in the DC link.